1. Field of the Invention
The present invention relates to a magnetic resonance imaging method for monitoring motion of a part of an object to be imaged by application of pulse and gradient sequences to the object, which sequences comprise a tagging section for producing a tagging pattern and an imaging section for producing magnetic resonance signals in the part of the object, the resonance signals being used for forming at least one image of the object. Such tagging patterns can be used in connection with cardiac imaging as so-called myocardial tagging grids to evaluate heart motion in multi heart phase imaging, but can also be used for other purposes such as radiation therapy port planning of a liver. In the latter case the tagging is quasi static.
The present invention further relates to a magnetic resonance imaging device for monitoring motion of a part of an object, which imaging device comprises means for generating a stationary homogeneous magnetic field, means for radiating radio frequency pulses to the object which is placed in the stationary field, gradient generating means for generating magnetic gradient fields which are superimposed upon the stationary field, and processing means which are arranged for applying pulse and gradient sequences to the object under the control of control means, the sequences comprising a tagging section and an imaging section, and which imaging device further comprises receiving means and sampling means for receiving and sampling of magnetic resonance signals generated by the sequences in at least a part of the object, the processing means being arranged for processing sampled resonance signals into at least one image.
Of interest is commonly owned copending application Ser. No. 07/859,749, filed Mar. 30, 1992 entitled "Magnetic Resonance Imaging Method and Device for Monitoring Motion of a part of an Object Based on Stimulated Echoes" filed concurrently herewith by the present inventors.
2. Description of the Prior Art
A method and device of this kind are known from the article, "MR Imaging of Motion with Spatial Modulation of Magnetization", L. Axel et at., Radiology, Volume 171, No. 3, 1989 pp. 841-845. For direct imaging of motion of the myocardium a multi heart phase imaging sequence is preceded by a pre-imaging so-called tagging sequence in order to obtain a line or grid reference pattern in the image. The tagging sequence is applied upon detection of the R-wave of an electrocardiogram. The multi heart phase imaging sequence produces magnetic resonance signals in a slice of the heart at different time intervals from the R-wave. Successive magnetic resonance images to be displayed visualize heart motion such as contraction and rotation, the tagging grid in the myocardium being displaced with respect to the chest wall tagging grid in successive heart images of the same slice. The tagging section of the imaging sequence comprises a radio frequency pulse to produce transverse magnetization, a magnetic field gradient to "wrap" the phase along the direction of the gradient and a further radio frequency pulse to mix the modulated transverse magnetization with the longitudinal magnetization. The resulting images show periodic stripes due to the modulation. A second set of modulation bands, for example, in a direction orthogonal to the first, can be produced by following the further radio frequency pulse with a further magnetic field gradient in an appropriate direction and then with a still further radio frequency pulse. Different tagging sections have also been described, e.g. for obtaining a radial tagging pattern, in "A Rapid Starburst Pulse Sequence for Cardiac Tagging", E. McVeigh et al., Book of Abstracts, SMRM 1989, Amsterdam, page 23, or in "Human Heart: Tagging with MR Imaging--A method for Noninvasive Assessment of Myocardial Motion", E. A. Zerhouni et al., Radiology, October 1988, pp. 59-63.
In all these methods it is difficult or not at all possible to evaluate the motion quantitatively, especially automatic evaluation using computer means, due to a relatively poor grid contrast. For images obtained at later stages of the heart cycle the contrast is reduced due to longitudinal relaxation effects. Not only does the grid pattern decay with longitudinal relaxation, but the image intensity becomes increasingly dominated by magnetization which has returned to its relaxed state, and which hence again becomes available for imaging. This relaxed magnetization which is devoid of the grid pattern, and which is thus not useful as far as the tagging is concerned, is a significant factor in reducing the grid contrast.